US Census Bureau
United States Census Bureau: Poverty in the Us [Pdf]
The US census report on poverty published in 2001.
University of Wisconsin
Institute for Research on Poverty: How Is Poverty Measured in the United States?
Describes how the United States measures poverty. Includes graphs comparing poverty rates over time, in different states, in different ethnic or age groups, etc.
US Census Bureau
U.s. Census Bureau: Poverty
Contains a wealth of information about poverty in the U.S. including statistics and characteristics of those living in poverty.
Other
The Feminization of Poverty: Past and Future
Very extensive report on the poverty trends among females in the U.S. and the reasons for these trends. Also contains information on recent and past poverty rates and ratios.